Conformational Changes and Decomposition Mechanisms of HMX-Based Cocrystal Explosives at High Temperatures

The cocrystallization effect plays a prominent role in improving the performance of energetic materials. A combinational strategy based on density functional tight-binding molecular dynamics (DFTB-MD) simulations and density functional theory (DFT) was used to elucidate the decomposition mechanisms and reaction kinetics of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)-based cocrystal explosives [HMX/N,N-dimethylformamide (DMF) and HMX/bis(2,4-dinitro-2,4-diazapentane) (DNDAP)] at high temperatures. The decomposition and reaction mechanisms of the two cocrystals showed great dependence on the temperature. In the HMX/DMF cocrystal, a conformational change of HMX at 2000 K and subsequent initial decomposition of HMX at 2500 K is involved. At 3000 K, the global decomposition and interaction of the HMX and cocrystal molecules occurred. There are two dominant competing reaction channels in the two cocrystals. The comparative results reveal that the HMX molecules have a larger reactivity with DNDAP at low temperatures but with DMF at high temperatures. The decomposition pathways of the HMX molecules based on DFT calculations were studied as a useful addition to the MD results. These findings provide a basic understanding of the thermal decomposition mechanisms and reaction kinetics of HMX-based cocrystal energetic materials at high temperatures.